val at mongo.mongobird.com
Sun Nov 29 06:23:37 PST 2009
I don't know for certain, but I would guess that the tire bouncing is
because the coeff. of friction is high enough to cause the revectoring
of momentum upwards with braking action. Kind of like chalk on a board.
It "sticks" and lifts, chattering.
The higher loaded weight causes a higher normal force and holds the
tires and road surface together (higher normal force), thereby increasing
the effective contact time, and minimizing the destabilized chatter.
Larger loads have higher inertia. Kinetic energy is proportional to the
square of the speed E(k) = 0.5 m v**2, so the total energy at 80 (pick
your velocity unit) is 4x the total energy at 40. The inertial effect
is linear...twice the weight is twice the total energy to dissipate.
There are some other factors we have not discussed on braking. Heat.
On some of the aircraft I fly, the brakes get hot enough with an aborted
takeoff, that there is a cooldown time, before a subsequent takeoff can
be accomplished. On some larger, transport aircraft, you can actually see
the stacked rotors glow.
Well, heat changes the coeff. of friction, and typically makes rotors
more plastic. From a functional standpoint, it is responsible for brake
fade. Therefore, braking effects are not linear with time of application
and recency of application.
In aircraft we used discs or stacked discs (rotors) because of weight
considerations, and the need for cooling with little airflow at taxi
speeds. A common operational problem with some aircraft is that extended
taxi and ground maneuvering can heat the brakes (they are often used
differentially for steering, or to augment steering, as well as limiting
ground speed) so that they must rest and cool prior to takeoff. Now
this doesn't happen every day, but it is a consideration. Thermocouples
in the brakes provide operational temps.
In larger trucks, I have always seen drums, which tend to have larger
thermal mass. That can hold off fade, and the mechanisms are not as
expensive to produce. However, once warm, they take longer to cool.
Perhaps one of the drivers in the group can comment on truck brake
fade. The sense I have (not having any large truck experience) is that
applying brakes on a significant downgrade, can leave things hot enough
to be noticable and readily impacts braking performance.
One more thing...some aircraft have a stainless steel rotor option. Not
real effective for braking, but for those who's planes sit outdoors
in corosive environments (coastal areas) and are infrequently used,
they have advantages.
Any railroad engineers in the group who want to talk about dynamic braking?
I suppose we'll go this way, when our 4 cyl TDIs get replaced with a
one banger, and some electric motors...
> If the tires start bouncing then of course that would change things.
> s it the heat that does that?
> It's the suspension, sort of. Tires grip and start to slide, no load on them so they want to
> stop sliding. Suspension is stiff so it's more like the whole trailer will then let inertia carry
> on while the tires are trying to stop and it'll then start hopping. Think motorcycle and
> stopping quickly with just the front brakes. The tractor has weight on the tires and
> stops faster than the trailer (my interpretation of what's going on anyway).
> My pappy always told me to never follow a partly loaded truck too close. They
> may seem like they can't stop fast but when you consider the square FEET of
> contact to the road compard to your car, they'll outstop you almost every time!
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